1. Field of the Invention
The invention relates to a method and a system for the detection and/or removal of sinusoidal interference signals in a noise signal.
2. Discussion of the Background
As illustrated in FIG. 1, measured noise signals conventionally comprise superimposed sinusoidal interference signals. The sources of these sinusoidal interference signals are to be found either internally within the device or externally. Some of the frequencies and interference lines are known in advance (network hum up to 10 harmonics, subsidiary lines of an internal frequency synthesiser, crosstalk from frequency dividers, microphone effects, for example, from fans) and some must still be determined.
These spectral lines associated with sinusoidal interference signals can be disturbing, for example, in the measurement of phase noise and must therefore be identified and removed as well as possible from the noise measuring curve. However, in some applications, it is also important just to measure the frequencies and power levels of the sinusoidal interference signals as accurately as possible.
In the context of a high-precision spectral analysis of a measured signal, it is therefore desirable to identify the individual spectral lines associated with the sinusoidal interference signals from the other spectral components of the measured signal, to measure the frequencies and power levels of the identified spectral lines and, if required, for example, in the case of a noise measurement, to remove the identified spectral lines completely from the measured spectrum.
According to the prior art, graphic methods are used for the identification of sinusoidal interference signals in a noise signal.
In one graphic method, as shown in FIG. 2, a threshold-value line is placed over the noise curve. All of the components of the spectrum disposed above this threshold-value line are recognised as spectral lines. The left and right intersection of the peak lines identified respectively by the threshold-value line with the peak-free noise curve is determined and linked by means of an appropriate interpolation.
The identification of spectral lines associated with sinusoidal interference signals in a noise spectrum requires a high-resolution Fourier Transform. Spectral lines disposed close together cannot be identified separately from one another using graphic means. The use of a constant threshold-value line is not appropriate in the case of a phase noise curve, which provides a monotonously descending course. Consequently, a threshold-value line must be used, which provides a course, which is constant only in very small regions, or a course which is complementary to the phase-noise curve. In such a case, measuring the position of spectral lines is disadvantageously still only possible in a heuristic manner. A further disadvantage of the graphic method is the fact that, because of the interpolation of the noise curve, the precise information about the noise curve in the region of the identified spectral lines is lost. Furthermore, with numerous spectral lines in the noise curve, the graphic imprint of the spectral curve is destroyed as a result of the numerous interpolations.